This invention relates generally to electrical propulsion systems used on traction vehicles (such as self-propelled rapid transit rail cars) to propel and retard the vehicle, the electric power input to the system being obtained from wayside conductors (e.g., a third rail) normally energized by direct current derived from electric power sources located at various stations along the right-of-way that the vehicle will travel, and it relates more particularly to means for detecting whether or not the wayside conductor with which the vehicle is in contact is so energized and for preventing any "regenerative" braking operation of the propulsion system if the wayside conductor is not otherwise energized.
A typical traction vehicle propulsion system comprises two pairs of electric traction motors, with the rotatable shafts of each pair being mechanically coupled through suitable gearing to the respective axle-wheels sets of a separate one of the two trucks that support the vehicle, two controllable electric power converters each having relatively positive and negative direct current (d-c) source terminals and a set of load terminals connected to a different pair of traction motors, a bi-directional current path including an electro-mechanical line switch or circuit breaker for connecting the d-c terminals of both converters to a set of current collectors protruding from the vehicle in sliding contact with a normally energized wayside source of unipolarity voltage having a relatively low, constant magnitude, and suitable control means for operating the converters either in a propulsion (motoring) mode when acceleration or constant speed of the vehicle is desired, or in an electrical retarding (braking) mode when deceleration is desired. Preferably the traction motors are three-phase alternating current (a-c) induction motors, the converters are three-phase voltage source inverters, and a low pass electrical filter is connected between the aforesaid line switch and the d-c terminals of the inverters.
In its motoring mode of operation, each inverter is so controlled that the unipolarity voltage applied to its source terminals is converted into three-phase alternating voltage of variable fundamental frequency and amplitude at its load terminals, and the a-c traction motors respond by producing torque to accelerate the vehicle or maintain its speed as desired. In the alternative electrical braking mode of operation, the inverter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the inverter and appears as direct current and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motors respond by absorbing kinetic energy and slowing the vehicle.
Electrical braking is achieved by a combination of dynamic and regenerative braking. Dynamic braking is effected by selectively connecting a dynamic braking resistance in parallel relationship with the d-c source terminals of the inverter. This resistance receives current from the inverter, converts the electrical energy to thermal energy, and dissipates the resulting heat. Regenerative braking, on the other hand, is effected by returning to the wayside source power flowing in a reverse direction through the inverter during electrical braking. The regenerated power can be advantageously utilized by the propulsion systems of other traction vehicles sharing the same wayside source of voltage and operating in their motoring mode. The two electrical braking modes can be combined in desired proportions, this mixing process being commonly referred to as "blending." An electrical propulsion system, including a voltage source inverter for supplying a-c traction motors, is disclosed in U.S. Pat. No. 4,904,918--Bailey, Kumar and Plette, granted on Feb. 27, 1990, and assigned to General Electric Company.
The wayside source of unipolarity voltage usually comprises two or more low-voltage d-c power generating plants or stations located near the right-of-way traveled by the traction vehicle. In a typical station the d-c power is derived from commercially available three-phase a-c electric power by means of a polyphase power transformer in combination with an uncontrolled power rectifying bridge having a set of a-c input terminals connected to the transformer secondary windings (i.e., the low voltage windings) and a pair of d-c output terminals across which the unipolarity voltage is produced. At each station one of the d-c terminals of the rectifying bridge (conventionally the one whose potential is negative with respect to the other d-c terminal) is grounded, and suitable means is provided for connecting the other terminal to a plurality of bare electrical conductors extending along different sections of the right-of-way. Proximate ends of the conductors in adjacent sections are separated from each other by relatively short, insulating gaps. Such gaps are commonly found at track crossings and switches and at other strategic locations along the route traveled by the vehicle. As the vehicle is driven by its electrical propulsion system along each different section of the right-of-way, its current collectors are in sliding contact with the corresponding conductor until a conductor gap is reached, at which point the vehicle will be unpowered for a relatively short distance (e.g., as short as four or five feet) until its leading current collector makes contact with the wayside conductor associated with the next section of the right-of-way. This external conductor is usually a third rail parallel to the pair of rails forming the track on which the vehicle travels, in which case the gaps are simply air gaps and the current collectors are spring-biased "shoes" respectively supported in cantilever fashion on the two trucks of the vehicle.
There are certain times when the wayside conductor in one section of the right-of-way will be temporarily disconnected from its normal voltage source for track maintenance work or for some other purpose. In this event, the wayside conductor is intended to be de-energized or "dead." If a vehicle enters such a de-energized section while its electric power converter is operating in a regenerative braking mode, there is a possibility that the regenerative current from the vehicle will raise the electrical potential on the wayside conductor to an undesirably high level, thereby endangering maintenance people who believe the conductor is dead. In order to prevent this hazard from occurring, suitable means for detecting whether or not the conductor is energized by a power generating station and for preventing regenerative braking if the conductor is not so energized are desired.
It is known in the prior art, as disclosed in U.S. Pat. No. 4,057,753, to provide a permissive control signal which is inserted into the electrical power supplied by each wayside generating station to the vehicle for establishing when regenerated power may safely be returned from the vehicle to the wayside conductor.
It is also known in the prior art, as described in U.S. Pat. No. 4,326,154, to open the line switch in the current .path between the electric power converter and the current collectors on board the vehicle as these current collectors traverse each wayside conductor gap every time the vehicle moves from section to section of the right-of-way, thereby preventing either motoring or regenerative braking operation of the propulsion system as the vehicle enters the next section. After the leading current collector makes contact with the wayside conductor of the next section, the line switch is not reclosed until voltage is detected on the current collectors and a current sensor indicates that appreciable current is flowing to auxiliary electrical load circuits on the vehicle.
The above-referenced prior art regenerative braking protective apparatus has shortcomings. If the propulsion system were operating in a regenerative braking mode and the traction vehicle were traveling at a relatively high speed (e.g., 40 MPH) as the current collectors pass through a gap between a first energized wayside conductor and a second de-energized conductor, the gap would be traversed in an interval of time (e.g., 70 milliseconds) that is shorter than a typical opening time (e.g., 100 milliseconds) of a conventional line switch. Consequently, an undesirable spike of high voltage could be applied to the de-energized conductor before the line switch has time to open the current path between the converter and the current collectors. Furthermore, if the wayside conductor were disconnected from its power generating station while in contact with a vehicle whose propulsion system is operating in a regenerative braking mode, the regenerative current would fool the protective apparatus so that neither the current-collector voltage detector nor the auxiliary load current sensor would cause the line switch to open.
It is known in the prior art relating to electrical propulsion systems for trolley buses to interrupt regenerative braking current in the bi-directional current path between each of two trolley poles and an electric power converter whenever the tandem trolley poles of the bus traverse insulator gaps in a pair of overhead power supply lines and thereafter to permit such current to be conducted to the power lines of the next section of right-of-way only if the unipolarity voltage across such lines has proper polarity and magnitude and is not decreasing. See U.S. Pat. No. 4,453,113 wherein the bi-directional path includes a diode bridge to ensure that the polarity of the voltage applied to the d-c source terminals of the converter during motoring operation will not change if line voltage polarity changes, a pair of the diodes are shunted by thyristors poled to conduct regenerative current, and a lightning arrestor capacitor is connected between the trolley conductors. During regenerative braking operation, both thyristors change from conducting to non-conducting states whenever the trolleys come to the insulator gap because current in the trolley conductors then decreases abruptly to zero and the resulting increase of voltage across the lightning arrestor capacitor puts a reverse bias on the thyristors. Later, the thyristors are returned to their conducting states in response to the concurrence of a number of conditions: the sensed line voltage has proper polarity and magnitude and is not decreasing; traction motor current exceeds a predetermined threshold magnitude; and the bus is moving faster than a predetermined speed.